[GHC] #12790: GHC 8.0.1 uses copious amounts of RAM and time when trying to compile lambdabot-haskell-plugins

[GHC]

#12790: GHC 8.0.1 uses copious amounts of RAM and time when trying to compile
lambdabot-haskell-plugins
-------------------------------------+-------------------------------------
        Reporter:  clint             |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by RyanGlScott):

 Great catch. Indeed, I cannot reproduce the issue anymore with GHC 8.2.2,
 GHC 8.4.1, or HEAD with the program in comment:7, and moreover, I can
 compile `lambdabot-haskell-plugins` on GHC 8.2.2 in a reasonable amount of
 time with profiling enabled.

 Here's what you get for the Core for `list` on GHC HEAD:

 {{{
 -- RHS size: {terms: 1, types: 0, coercions: 7, joins: 0/0}
 list :: Parser Expr
 list
   = Parser.list1
     `cast` (Sym (Text.Parsec.Prim.N:ParsecT[0]
                      <[Char]>_R <()>_R <Data.Functor.Identity.Identity>_R
 <Expr>_R)
             :: (forall b.
                 Text.Parsec.Prim.State [Char] ()
                 -> (Expr
                     -> Text.Parsec.Prim.State [Char] ()
                     -> Text.Parsec.Error.ParseError
                     -> Data.Functor.Identity.Identity b)
                 -> (Text.Parsec.Error.ParseError
                     -> Data.Functor.Identity.Identity b)
                 -> (Expr
                     -> Text.Parsec.Prim.State [Char] ()
                     -> Text.Parsec.Error.ParseError
                     -> Data.Functor.Identity.Identity b)
                 -> (Text.Parsec.Error.ParseError
                     -> Data.Functor.Identity.Identity b)
                 -> Data.Functor.Identity.Identity b :: *)
                ~R# (Text.Parsec.Prim.ParsecT
                       [Char] () Data.Functor.Identity.Identity Expr :: *))

 -- RHS size: {terms: 8, types: 32, coercions: 0, joins: 0/0}
 Parser.list1
   :: forall b.
      Text.Parsec.Prim.State [Char] ()
      -> (Expr
          -> Text.Parsec.Prim.State [Char] ()
          -> Text.Parsec.Error.ParseError
          -> Data.Functor.Identity.Identity b)
      -> (Text.Parsec.Error.ParseError
          -> Data.Functor.Identity.Identity b)
      -> (Expr
          -> Text.Parsec.Prim.State [Char] ()
          -> Text.Parsec.Error.ParseError
          -> Data.Functor.Identity.Identity b)
      -> (Text.Parsec.Error.ParseError
          -> Data.Functor.Identity.Identity b)
      -> Data.Functor.Identity.Identity b
 Parser.list1
   = \ (@ b_X3Bv)
       _ [Occ=Dead]
       _ [Occ=Dead]
       _ [Occ=Dead]
       _ [Occ=Dead]
       _ [Occ=Dead] ->
       case Parser.list2 of wild_00 { }
 }}}

 Hooray!

 I'll add this as a test case, since GHC HEAD includes `parsec` as a boot
 library nowadays.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12790#comment:17>
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